Vehicle internal combustion engine belt drives normally feature a tensioner for ensuring correct tension of the drive belt. In the case of complex drives, and when space is limited, elongated torsion bar tensioners are preferred to conventional types, by being more compact transversely, and therefore housable alongside the engine or an engine accessory.
Patent DE A 3 912 944 describes a torsion bar tensioner comprising an elongated protective tube; an arm rotating about a longitudinal axis of the protective tube and supporting an idle pulley at the end; a torsion bar defined by a group of three elastic beam elements, which exert elastic thrust on the arm to ensure correct tension of the drive belt; and a damping device for damping angular oscillation of the arm.
More specifically, the elastic elements have a hexagonal cross section, and the protective tube housing the elastic elements is connected rigidly to the arm at a first end, and cooperates with the damping device at a second end opposite the first.
The damping device comprises a truncated-cone-shaped housing integral with a fixed wall of the engine; a friction member connected prismatically to the protective tube and cooperating frictionally with an inner work surface of the truncated-cone-shaped housing; and a Belleville washer for pressing the friction member against the work surface.
In actual use, the protective tube is housed to rotate freely inside a supporting bush, the arm tightens the belt by means of the pulley, and oscillation of the arm transmitted to the protective tube is damped by the damping device.
The damping device described above has the drawback of being connected to a wall of the engine, and of comprising a large number of component parts, thus complicating assembly and increasing the length of the device.
Moreover, the Belleville washer, being interposed between the fixed wall and the friction member integral with the arm, is subject to wear, and must therefore be sized to ensure an adequate axial load even when the friction member is worn.